ACP Insights & Guidance
Soakaway testing helps a drainage designer assess whether the ground can receive surface-water runoff. Agree the investigation brief before fieldwork, including the proposed location, depth, groundwater evidence and design decision.
The practical answer
A measured infiltration rate is an input to design, not approval of a drainage solution. Let the designer define representative test locations and the applicable method. Coordinate groundwater, contamination, access and reporting before commissioning the visit.
What should soakaway testing establish?
Begin by distinguishing feasibility from detailed design. An early investigation may help compare drainage options. A later investigation should support the actual feature locations and levels. Tell the field team which decision its work needs to inform.
In England, Defra’s non-statutory SuDS standards require suitable ground conditions and appropriate infiltration evidence. They also address groundwater separation, pollution and instability risks. Therefore, the drainage designer needs more than a single rate copied into a calculation.
Coordinate this work with the wider flood risk assessment brief. Discharging runoff into the ground does not remove the need to understand other flood sources or how the site manages exceedance.
Choose locations and depths with the designer
First, provide the current site plan and proposed drainage areas. Ask the designer to identify the locations and depths needed to investigate the intended solution. A convenient test position near the entrance may tell you little about a feature at the opposite end of the site.
Next, consider variability. Made ground, different geological layers or changes in site levels may justify separate investigation areas. The appointment should explain how the team will respond if the encountered ground differs from the expected conditions.
Also identify the reference level used in the records. Ground level, test depth and proposed base level should be traceable to the same design information. Otherwise, the designer may struggle to relate a useful measurement to the intended drainage feature.
Information to agree before soakaway testing
| Topic | Provide or agree | Why it matters |
|---|---|---|
| Design | Current layout, proposed features, levels and the investigation objective. | Connects fieldwork to the drainage decision. |
| Method | Applicable test method, locations, depths, repeats and reporting scope. | Avoids an ambiguous appointment or unusable record. |
| Site constraints | Access, services information, contamination history and reinstatement requirements. | Supports safe and practical fieldwork planning. |
| Water and conditions | Water supply, groundwater evidence and observations to record. | Clarifies resources and limitations before mobilisation. |
| Decision output | Factual logs, calculations, interpretation and the designer’s review. | Separates measured evidence from design conclusions. |
BRE 365: agree the applicable method before fieldwork
Where BRE Digest 365 forms part of the agreed method, identify the applicable edition and requirements in the appointment. Do not treat a generic ‘percolation test’ description as proof that the work will meet the drainage designer’s needs.
The brief should also allocate responsibility for excavation, water supply, service checks and reinstatement. These practical arrangements can determine whether the field team completes the planned work or returns with partial results.
Treat groundwater and contamination as separate evidence questions
A dry excavation on one visit does not establish the highest likely groundwater level. Ask what seasonal groundwater evidence the design needs and how that evidence will be obtained. Record the observation date and conditions rather than describing a single visit as a complete groundwater assessment.
Similarly, infiltration can alter how water moves through potentially contaminated ground. Coordinate the drainage proposal with the contaminated land assessment so the specialists can consider relevant pollutant pathways.
Do not interpret this as a blanket prohibition on infiltration at every previously developed site. The issue is whether the evidence supports the proposed location and solution. Where uncertainty remains, identify the additional assessment or alternative design work needed.
Read the results as a design evidence package
A useful report identifies the test positions, ground conditions, dimensions, water observations and measurement records. It should state the method and any departures. The drainage designer can then judge how representative the results are for the proposed features.
Ask the designer to explain the selected design input and any uncertainty allowance under the applicable method. Keep the factual measurements separate from those design choices. This makes later review easier if the layout or feature depth changes.
If the report contains an incomplete or unsuccessful test, record what happened and what it means for the decision. ‘No usable result’ and ‘infiltration is unsuitable throughout the site’ are different conclusions. Further evidence may be needed before selecting the drainage route.
Example: a layout change after the investigation
Illustrative design coordination
A team investigates an infiltration area beside a proposed car park. Later, the layout moves the drainage feature into a landscaped strip at a different level. The original report still contains valid measurements, but they relate to the original positions and depths.
The project manager asks the drainage designer whether those results remain representative. The designer reviews the ground information and decides whether further testing is needed. The team records that decision before finalising the revised layout.
This example illustrates evidence management. It does not prescribe a testing density, infiltration rate or soakaway size for a real site.
Turn the test findings into clear next actions
- Confirm whether the investigation answered the agreed design question.
- Identify limitations and any further ground or groundwater evidence.
- Record the drainage option selected and who made the decision.
- Coordinate feature levels with buildings, utilities, trees and landscaping.
- Check maintenance access and how the system handles blockage or exceedance.
- Update the evidence if the proposed infiltration area materially changes.
Finally, keep the investigation report with the drawing revision it informed. This simple link helps the planning, design and construction teams avoid reusing a result outside its original context.
Common questions
Does a soakaway test prove that planning permission will be granted?
No. It provides evidence for a drainage decision. Planning requirements, flood risk, pollution, ground stability and other relevant matters remain part of the wider assessment.
Can testing happen before the final layout is ready?
Yes, early work can inform feasibility. However, the designer should explain its scope and whether later locations or levels may require further evidence.
Is a dry trial pit enough to assess groundwater?
No. It is one observation at a particular time and place. The designer should establish what evidence is needed to understand relevant seasonal groundwater conditions.
Can we reuse tests from another part of the site?
Only if the responsible designer considers them representative and explains the basis. Differences in ground, depth or proposed design can make additional investigation necessary.
What happens if infiltration is unsuitable?
The drainage team should assess other feasible runoff destinations and design options under the applicable requirements. An alternative outfall may need separate agreement or consent.
What affects soakaway test cost?
The investigation scope, access, locations and depths, water supply, reinstatement and reporting all matter. Compare like-for-like quotations and identify the cost basis for extra visits or incomplete tests. A fee alone does not tell you whether the work meets the designer’s brief.
References and scope
England-focused commissioning guide. It does not reproduce BRE Digest 365, calculate infiltration rates or specify a project’s fieldwork. The appointed specialists must agree the current method and site-specific requirements.
- Defra. National standards for sustainable drainage systems (SuDS), Standards 1, 3 and 4.England; non-statutory standards. Relevant to ground suitability, investigation, groundwater and pollution considerations.
- Environment Agency. Land contamination risk management: Stage 1 risk assessment.Risk-based assessment framework used to explain coordination with contamination evidence, rather than prescribe a drainage test method.
- BRE. Soakaway design, Digest 365, revised 2016.Publisher catalogue confirms edition and scope. Full licensed text was not reviewed; no test procedure or calculation is reproduced.